Solar Cycle Prediction

Abstract

A review of solar cycle prediction methods and their performance is given, including forecastsfor cycle 24. The review focuses on those aspects of the solar cycle prediction problem thathave a bearing on dynamo theory. The scope of the review is further restricted to the issue ofpredicting the amplitude (and optionally the epoch) of an upcoming solar maximum no laterthan right after the start of the given cycle.

Prediction methods form three main groups. Precursor methods rely on the value of somemeasure of solar activity or magnetism at a specified time to predict the amplitude of thefollowing solar maximum. Their implicit assumption is that each numbered solar cycle is aconsistent unit in itself, while solar activity seems to consist of a series of much less tightlyintercorrelated individual cycles. Extrapolation methods, in contrast, are based on the premisethat the physical process giving rise to the sunspot number record is statistically homogeneous,i.e., the mathematical regularities underlying its variations are the same at any point of timeand, therefore, it lends itself to analysis and forecasting by time series methods. Finally, insteadof an analysis of observational data alone, model based predictions use physically (more or less)consistent dynamo models in their attempts to predict solar activity.

In their overall performance during the course of the last few solar cycles, precursor methodshave clearly been superior to extrapolation methods. Nevertheless, most precursor methodsoverpredicted cycle 23, while some extrapolation methods may still be worth further study.Model based forecasts have not yet had a chance to prove their skills. One method that hasyielded predictions consistently in the right range during the past few solar cycles is that ofK. Schatten et al., whose approach is mainly based on the polar field precursor.

The incipient cycle 24 will probably mark the end of the Modern Maximum, with the Sunswitching to a state of less strong activity. It will therefore be an important testbed for cycleprediction methods and, by inference, for our understanding of the solar dynamo.